1. Field of the Invention
The present invention relates generally to microlens arrays and optical lenses, and more particularly to methods for manufacturing microlens arrays or non-spherical lenses.
2. Related Art
Microlens arrays provide optical versatility in a miniature package for imaging applications. Traditionally, a microlens is defined as a lens with a diameter less than one millimeter; however, a lens having a diameter as large as five millimeters or more has sometimes also been considered a microlens.
There are many conventional methods for manufacturing microlens arrays, such as using reflow or diffusion. FIGS. 1A–1C show a typical sequence of steps for making a microlens array by depositing material, patterning, and reflowing. In FIG. 1A, a photosensitive layer 10, such as a photosensitive resin, is formed on a planarization layer 12 over a silicon substrate (not shown). The material of the photosensitive layer is used to form the microlens array. In FIG. 1B, photosensitive layer 10 is patterned to form an array of shapes, such as rectangles, stripes, or squares 14, where the shapes are located where the individual microlenses will be formed. Patterning, for example, can be with a conventional mask and photoresist process, where a photoresist is deposited on photosensitive layer 10, exposed through a mask having opaque areas, developing (or removing) selected portions of the photoresist, and etching areas of photosensitive layer 10 left exposed by the photoresist. Squares 14 are then heated sufficiently to cause them to reflow, thereby forming an array of semi-spherical microlenses 16, as shown in FIG. 1C.
However, microlens arrays made by thermal reflowing, as described above, have several disadvantages. Typically, photosensitive resins contain components which absorb proportionally more light in the blue region of the visible spectrum. As a result, the color spectrum is distorted, producing an image that is more “yellowish” than it should be. This color distortion increases with time due to oxidation of the resin. Another disadvantage is that the resolution with which the photosensitive resin can be patterned is limited by the thickness of the resin layer. The thicker the resin layer, the farther apart the microlenses in the array, which reduces the light collection efficiency of the array. On the other hand, the resin layer must be thick enough so that, when reflowed, the sag of the resultant microlenses is sufficient to cause the desired focusing effect. Consequently, it is difficult to obtain the highest possible collection efficiency with microlens arrays fabricated in this manner. Yet another disadvantage results from the fact that as the curvature radius of the microlens becomes small, the incident light is focused on a point near the microlens. Thus, the photosensitive layer is patterned to be square or rectangular in shape according to the shape of a cell, using a mask that is simply divided into opaque regions and light-transmissive regions, and is thermally treated to form a microlens. Thus, a curvature radius of the microlens is decreased. Moreover, because a microlens formed in a rectangular shape has a significant difference between its curvature radius in the width and the length directions, it is difficult to focus incident light on the corresponding photodiode without error, and a part of the light is focused on the planarization layer or color filter layer between the photodiode and the microlens, causing loss of light and deterioration of sensitivity and resolution.
Another conventional method of forming microlens arrays is by diffusion, such as described in “Light Coupling Characteristics of Planar Microlens”, by M. Oikawa et al., Proc. SPIE, 1544, 1991, pp. 226–237, which is incorporated by reference in its entirety. FIGS. 2A–2G show steps for forming a microlens array using two types of diffusion. In FIG. 2A, a glass substrate 20 is provided. In FIG. 2B, a metal film 22 is deposited on glass substrate 20. Metal film 22 is then patterned, such as with conventional processes, to remove portions 24 where individual microlenses are to be formed, as shown in FIG. 2C. FIGS. 2D and 2E show one type of further processing, where the exposed areas 24 are diffused with an appropriate dopant and energy (FIG. 2D) and then the remaining metal is removed and the surface is polished, such as with a chemical or machine polish, to form microlenses 26 (FIG. 2E). FIGS. 2F and 2G show another type of further processing, where ions, protons, or other suitable molecules are used to bombard (e.g., with low energy) (FIG. 2F) and diffuse into substrate 20 and the remaining metal portions removed and the irradiated portions “swelled” (FIG. 2G), such as with an organic vapor, to form microlenses 28. The result is a high numeral aperture planar microlens array. One disadvantage to forming microlens arrays using diffusion is that control of the thickness along the optical axis is limited.
Microlens arrays are typically used with an underlying array of sensors, such as complementary metal oxide semiconductor (CMOS) or charge couple device (CCD) sensors, to form an imaging device. The microlenses collect and focus light onto corresponding sensors. The microlenses significantly improve the light sensitivity of the imaging device by collecting light from a large light collecting area and focusing it on a small light sensitive area of the sensor (i.e., pixel). One conventional method of generating an image signal is shown in FIG. 3. Light rays 30 are collected and focused by a microlens layer 32 comprising an array of microlenses 34 overlying a planarization layer 36, such as formed by processes described above. After passing through planarization layer 36, light rays 30 are filtered by color filters 38 in a filter layer 40, with each color filter allowing only light of a specific color to pass, such as red, green, and blue (RGB). Light through the filters are then passed through a sensor layer 42, comprising an array of sensors 44, such as photodiodes or CCD devices. A processor (not shown) combines signals from the sensors to create a color image.
Such an arrangement of microlenses, filters, and sensors has several disadvantages. Several processing steps are needed to form the separate microlens layer 32, filter layer 40, and sensor layer 42, which increase cost and time. The layers also increase the separation between the microlenses and the sensors, which can increase crosstalk between pixels, due in part to light impinging on adjacent sensors instead of the desired sensor.
In addition to microlenses, high quality non-spherical lenses are also critical components to many applications in the imaging field. They are widely used in optical systems for controlling critical light propagation and correcting image color quality, such as in professional cameras and video imaging equipment. However, the fabrication of non-spherical lenses is complicated and can only be done through skilled manual operation by highly trained professionals. Unlike spherical lenses which can be manufactured quickly by using conventional machines, non-spherical or specially sized or shaped lenses are typically shaped and polished manually and frequently individually. This can be time consuming and costly.
Accordingly, there is a need for an improved lens, microlens, or array and method of manufacturing such, including non-spherical lenses, that overcomes the disadvantages of conventional lens arrays or non-spherical lenses and related processes, such as described above. Further, there is a need for an integrated microlens array and sensor array that overcomes the disadvantages as described above with conventional microlens/sensor devices.